DNA sequencing

ABSTRACT

To sequence long strands of DNA, cloned strands having lengths longer than 100 bases are, in one embodiment, marked on one end with biotin. These strands are divided into 4 aliquots and each aliquot: (1) is uniquely chemically treated to randomly terminate the strands at the non-biotinylated end at a selected type of base; and (2) is moved continuously by electrophoresis through a different one of four identical channels. In the one embodiment, the strands are randomly terminated at a selected base type and they are moved into avidin, which due to high affinity, combines with the biotin marked ends of shorter strands before the longer strands are fully resolved in the gel. The avidin is marked with fluorescein, the strands are scanned and the signals are decoded. In another embodiment, the strands are synthesized, with termination at a selected base type and marked either by the above method or by ethidium bromide.

BACKGROUND OF THE INVENTION

This invention relates to the sequencing of DNA strands.

In one class of techniques for sequencing DNA, identical cloned strandsof DNA are marked. The strands are separated into four batches andeither individually cleaved at or synthesized to one of the four basetypes, which are adenine, guanine, cytosine and thymine (hereinafter A,G, C and T). The adenine-, guanine-, cytosine- and thymine- cleavedbatches are then electrophoresed for separation. The rate ofelectrophoresis indicates the DNA sequence.

In a prior art sequencing technique of this class, the DNA strands aremarked with a radioactive marker, cleaved at a different base type ineach aliquot, and after being separated by electrophoresis, film isexposed to the gel and developed to indicate the sequence of the bands.The range of lengths and resolution of this type of static detection islimited by the size of the apparatus.

In another prior art sequencing technique of this class, single strandsare synthesized to a different base type in each aliquot, and thestrands are marked radioactively for later detection.

It is also known in the pirior art to use fluorescent markers formarking proteins and to pulse the fluorescent markers with light toreceive an indication of the presence of a particular protein from thefluorescence.

The prior art techniques for DNA sequencing have several disadvantagessuch as: (1) they are relatively slow; (2) they are at least partlymanual; and (3) they are limited to relatively short strands of DNA.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a noveltechnique for DNA sequencing.

It is a still further object of the invention to provide novel apparatusand methods for sequencing relatively large chains of DNA.

It is a still further object of the invention to provide apparatus andmethods for sequencing cloned DNA fragments of 100 bases or more.

It is a still further object of the invention to provide a technique forcontinuous sequencing of DNA.

It is a still further object of the invention to continuously sequenceDNA without the spatial limitations of range of lengths and resolution.

It is a still further object of the invention to provide a technique forsequencing of DNA.

It is a still further object of the invention to provide a noveltechnique for continuously sequencing DNA using fluorescent detection.

It is a still further object of the invention to provide a noveltechnique for DNA sequencing using a fluorescent marker attached to theDNA, or the inherent fluorescence of the DNA itself.

It is a still further object of the invention to provide a noveltechnique for continuously sequencing DNA marked with fluorescence whichmore clearly distinguishes marked DNA fragments from backgroundfluorescent noise.

It is a still further object of the invention to provide a noveltechnique for continuously sequencing DNA using radioactive detection.

In accordance with the above and further objects of the invention, oneembodiment of apparatus for sequencing DNA includes at least fourelectrophoresis channels each adapted to receive cloned DNA strandslabeled at one end with biotin and cleaved at the other end at a giventype of base. Each of the channels has a gel path and electrical fieldacross it identical in its characteristics to the gel path of the otherchannels and electrical fields across the other channels.

To provide marking, means are provided for introducing biotin into theDNA fragments prior to their being electrophoresed into the gel with thegel and field being selected so that strands being electophoresedtowards the terminal end of the gel channel are fully resolved prior tothe resolution of longer strands towards the beginning of the channel,and so on, in a continuous process over a period of time.

At the terminal end of this separating gel, there is provided means forapplying avidin to the strands to further mark the strands individuallywhile maintaining the strands in each channel separate from the strandsin other channels. The avidin is pre-marked with multiple fluorescentmolecules and therefore provides multiple fluorescent markers for eachseparated strand. The application of avidin to the strands may be duringfurther electrophoresing in a second section of the gel, in whichunattached avidin is stationary, but the fluorescein- avidin- biotin-DNA complex continues to move.

In another embodiment, strands are synthesized with primers whichprimers are an inverted complementary sequence. These primers aresynthesized by a DNA synthesizer available commercially such as thatmanufactured by Applied Biosystems. After separation, the invertedcomplementary sequencne forms a hairpin in which ethidium bromideintercalates.

In another embodiment, after separation, ethidium bromide intercalatesin duplex DNA formed by alidromes of unprimed DNA or in the singlestranded DNA.

In another embodiment, the inherent fluorescence of DNA may be used as asuitable detection mechanism. Thus, it is not necessary to mark one endof the strands with biotin nor mark them with fluorescein nor attachprimers with inverted complementary sequences.

In another embodiment, radioactive markers attached directly to DNA maybe used as a suitable detection mechanism.

The gel electrophoresis may be provided in conventional gel slabs withinput sections for each of the four channels for A, G, T and C, inaddition to any timing that may be needed. As alternatives (1) fourchromatography tubes may be used with gel in them so as to provide moreuniform temperature control and eliminate the need for timing channels;(2), open capillary tubes may be used and thus avoid the need for geland make the cleaning more convenient; or (3), high performance liquidchromatography (HPLC) columns such as ion-exchange columns or reversephase columns may be used in conjunction with high pressure instead ofhigh voltage for separating the strands within each channel or batch. Inusing HPLC, sequencing would be performed on smaller strands of DNAcalled oligonucleotides with typical lengths of 10-50 bases, using onecolumn for each aliquot or at least four columns.

The detection of the strands is accomplished by moving the strands bybulk flow after electrophoresis or HPLC separation while scanning themwith a source of light. Means are provided for detecting the bandsindividually from each channel in accordance with their time of exitfrom the gel to indicate the sequence of the A, G, C and T strands ofdifferent lengths. Advantageously, an additional channel may be utilizedas a calibration channel through the electrophoresis of DNA strands ofknown, but different lengths. These DNA strands are also marked andthereby indicate a time base.

The scanning apparatus includes a light source, such as a laser ormercury-arc lamp or other suitable source, which emits light in theoptimum absorption spectrum of the marker. The light may be split by theuse of fiber optics or other conventional optical components, so thatthere is a source for each of the 4 sample channels as well as anycalibration channels.

The detector includes a filtering system for passing selectively theoptimum emission band of the fluorescent marker to a light sensor whichis preferably a photomultiplier. The photomultiplier or other lightcontrolled mechanism selectively detects the fluorescence usingtechniques which enhance the signal/noise ratio. One technique is theuse of laser pulses which are less than five nanoseconds time duration,with detection in a time window. The length of such window and its delayfrom the pulse are optimized to discriminate against backgroundfluorescence. Another technique is to modulate the laser source with anelectro-optic modulator, with detection by a lock-in amplifier. There isa detector for each channel, and the combination thereof, indicates: (1)if the type of base termination or nucleotide cleavage is A, G, C or T;and (2) the time of emergence of each strand from each channel of theelectrophoresis gel or HPLC column to indicate the overall sequence ofstrands.

To use the apparatus to sequence DNA strands, cloned strands arenormally formed of a length greater than 100 bases. In one embodiment,the strands are marked by biotin at one end. The strands are dividedinto four aliquots and the strands within each aliquot are cleaved at adifferent base type. In another embodiment, strands are synthesized toselected base types. These four batches are then electrophoresed throughidentical channels to separate strands such that the shorter strands areresolved towards the end of the gel prior to resolution of the longerstrands, which still are near the beginning of the gel. This occurs in acontinuous process so a substantial number of different length strandsmay be resolved in a relatively short gel. This methodology takesadvantage of time-resolved bands, as opposed to the limitations ofspatial-resolved bands.

The gel size, electric field and DNA mobilities are such that the firstbands to be moved completely through the gel are fully resolved whilethe last bands are yet unresolved in a continuous process such that atleast ten percent of the bands are resolved and electrophoresed throughthe gel while the lesser mobile bands are yet unresolved near theentrance end of the gel. These lesser mobile bands become resolvedlittle by little over time in a continuous fashion without interruptionof the movement of these bands through the gel.

In one embhodiment, near the end of the gel, the biotin terminatedfragments are further combined with avidin. The avidin, being arelatively large molecule, may have a plurality of fluorescent markersfor each avidin molecule to provide signal amplification. Thecombination of biotin and avidin may take place either within a secondsection gel or in liquid after the bands leave the gel.

To attach the avidin within the second section of the gel, the pH ofthis section may be different from that of the first section. In such agradient gel the biotin-marked strands contact the avidin duringelectrophoresis. Marked avidin is stationary at a gel pH that isdependent on the number of fluorescein molecules attached to it, whereasDNA is mobile at a gel pH above 4. The electrophoresis of the DNA isdone in a first section of the gel having a pH of approximately 7-8,while a band of avidin is located in a second section having its pH inwhich the fluorescein marked avidin is stationary. In the preferredembodiment, three fluorescein markers are used for each molecule ofavidin and the fluoresceinated-avidin has a pI of approximately 8. Theavidin should be pure and not contain any DNA or else non-specificstaining may occur. The distance to the second section is sufficientlylong enough so that the DNA strands are resolved into bands beforereaching the avidin.

The markers are detected by transmitting light in the one embodiment tothe fluorescent-avidin-biotin-DNA complexes, in another embodiment tothe ethidium-bromide-DNA hairpin complex, and in another embodiment, toan ethidium bromide unmarked DNA complex and in yet another embodiment,to plain DNA, using wavelengths in a narrow wavelength bandwidth in theoptimum absorption spectrum of the markers on DNA and detecting emittedfluorescent light either during a time period in which the markers'fluorescence has not yet decayed to an insignificant amount but thebackground fluorescence has or by modulating the light source anddetecting using lock-in techniques. The detection is made in awavelength band including at least as a principal portion of its energy,the high emission spectrum of the fluorescent marker. For the gatedwindow technique, the light is transmitted from pulsed lasers inapproximately three nanosecond pulses. Readings are taken within awindow period, after an initial delay, both period and delay areoptimized for best results.

In another embodiment, radioactive marked strands, after beingseparated, are combined with scintillation liquid whereby detection ofthe presence of the strands is accomplished by an appropriatephotodetector.

From the above summary, it can be understood that the sequencingtechnique of this invention has several advantages, such as: (1) ittakes advantage of resolution over time, as opposed to space, (2) it iscontinuous; (3) it is automatic; (4) it is capable of sequencingrelatively long strands including strands of more than 100 bases; and(5) it is relatively economical and easy to use.

SUMMARY OF THE DRAWINGS

The above noted and other features of the invention will be betterunderstood from the following detailed description when considered withreference to the accompaning drawings in which:

FIG. 1 is a block diagram of an embodiment of the invention;

FIG. 2 is a block diagram of another embodiment of the invention;

FIG. 3 is a simplified schematic of a portion of the embodiment of FIGS.1 and 2;

FIG. 4 is an alternative embodiment of the portion of FIG. 3;

FIG. 5 is another alternative embodiment of the portion of FIG. 3;

FIG. 6 is a block diagram of a portion of the embodiments of FIGS. 1 and2;

FIG. 7 is a logical circuit diagram of a portion of the block diagram ofFIG. 3; and

FIG. 8 is a schematic circuit diagram of a portion of the embodiments ofFIGS. 1 and 2.

DETAILED DESCRIPTION

In FIG. 1, there is shown a block diagram of a DNA sequencing system 10having a biotin labeling system 11, a DNA cleavage system 12, aseparating system 14, a detection and processing system 16 and a sourceof standard length DNA 18. Biotin labeling takes place before dividingthe DNA cloned strands into 4 aliquots.

The biotin from any suitable commercial source is added to the clonedstrands of more than 100 bases in a container as indicated at 11. Thebiotin preparation must be sufficient to mark at least one end of asubstantial proportion of the DNA fragments with the biotin in a mannerknown in the art.

Biotin is selected because of its affinity to avidin and becuase it isnot a large molecule, which in the latter case when added to the DNAfragments might substantially dominate the mobility of the DNA fragmentsduring electrophoresis. Being a small molecule, it does not prevent thediscrimination between different DNA fragments within the separatingsystem 14.

Although biotin has been selected as a marker which may be combinedlater with a larger molecule such as avidin, other markers may be used.They must have characteristics which enable them to be attached to a DNAfragment and to have a strong affinity to a larger molecule which may bemarked with a fluorescein or other suitably detectable material. Theymust also be of such a size and have such chemical characteristics tonot obscure the normal differences in the mobilities between thedifferent fragments due to cleavages at different ones of the adenine,guanine, cytosine and thymine bases.

In addition, a radioactive marker such as radioactive phosphorus orradioactive sulfur, radioactive carbon or tritium may be incorporatedinto the DNA molecules such that after separation, strands are combinedwith scintillation liquid.

The DNA cleavage system 12 communicates in four paths and the source ofstandard length DNA 18 communicates in one path within the separatingsystem 14 to permit passage of DNA fragments and standard fragmentsthereto in separate paths. The separating system 14, which sequencesstrands by separation, communicates with the detection and processingsystem 16 which analyzes the fragments by comparison with each other andthe standard from the source of standard length DNA 18 to deriveinformation about the DNA sequence of the original fragments.

The DNA cleavage system 12 includes four sources 20A, 20G, 20C, 20T offragments of the same cloned DNA strand. This DNA strand is normallygreater than 100 bases in length and is then further cleaved by chemicaltreatment to provide different lengths of fragments in each of fourcontainers 20A, 20G, 20C and 20T.

In one embodiment, the container 20A contains fragments of DNA strandsrandomly cleaved by a chemical treatment for A; the container 20Gcontains fragments of DNA strands randomly cleaved by a chemicaltreatment for G; container 20C contains fragments of DNA strandsrandomly cleaved by a chemical treatment for C; and container 20Tcontains fragments of DNA strands randomly cleaved by a chemicaltreatment for T. Thus, identical fragments in each container have beencleaved at different bases of a given base type by the appropriatechemical treatment.

The fragments in the containers are respectively referred to as A-DNAfragments, G-DNA fragments, C-DNA fragments and T-DNA fragments from thecontainers 20A, 20G, 20C and 20T respectively. These fragments areflowed from the containers 20A, 20G, 20C and 20T through correspondingones of the conduits 22A, 22G, 22C and 22T into contact with theseparating system 14.

The source of standard length DNA 18 includes a source of reference DNAfragments of known but different lengths which are flowed through aconduit 22S to the separating system 14. These reference fragments haveknown lengths and therefore their time of movement through theseparating system 14 forms a clock source or timing source as explainedhereinafter. While in the preferred embodiment the cloned strands of 100bases are marked with biotin before being divided into four batches,they may be marked instead after dividing into four batches but beforethe selected chemical treatment.

The separating system 14 includes five electrophoresis channels 26S,26A, 26G, 26C and 26T. The electrophoresis channels 26S, 26A, 26G, 26Cand 26T include in the preferred embodiment, gel electrophoresisapparatus with each path length of gel being identical and having thesame field applied across it to move samples continuously through fivechannels. The gels and fields are selected to provide a mobility to DNAstrands that does not differ from channel to channel by more than 5% invelocity. In addition, the field may be varied over time to enhance thespeed of larger molecules after smaller molecules have been detected, aswell as to adjust the velocities in each channel based on feedback fromthe clock channel to compensate for differences in each channel suchthat the mobilities in each channel are within the accuracy required tomaintain synchronism among the channels.

Preferably the gels are of the same materials, chemical derivatives andlengths and the electric fields are within 5% of the intermediates ofeach other in each channel. However, more than one reference channel canbe used such that a reference channel is adjacent to a sample channel inorder to minimize the requirements for uniformity of DNA movement in thegel for all channels.

The electrophoresis channel 26S receives fragments of known length DNAmarked with biotin and moves them through the gel. Similarly, each ofthe electrophoresis channels 26A, 26G, 26C and 26T receivesbiotin-labeled fragments from the cleavage system 20A, 20G, 20C and 20Tand moves them in sequence through the sample electrophoresis channels,with each being moved in accordance with its mobility under a fieldidentical to that of the reference electrophoresis channel 26S.

To provide information concerning the DNA sequence, the detection andprocessing system 16 includes five avidin sources 30S, 30A, 30G, 30C and30T; five detection systems 32S, 32A, 32G, 32C and 32T and a correlationsystem 34. Each of the avidin sources 30S, 30A, 30G, 30C and 30T isconnected to the detecting systems 32S, 32A, 32G, 32C and 32T. Each ofthe outputs from corresponding ones of the electrophoresis channels 26S,26A, 26G, 26C and 26T within the separating system 14 is connected to acorresponding one of the detection systems 32S, 32A, 32G, 32C and 32T.In the detection system, avidin with fluorescent markers attached andDNA fragments are combined to provide avidin marked DNA fragments withfluorescent markers attached to the avidin to a sample volume within thedetection system for the detection of bands indicating the presence orabsence of the fragments, which over time relates to their length.

The output from each of the detection systems 32S, 32A, 32G, 32C and 32Tare electrically connected through conductors to the correlation system34 which may be a microprocessor system for correlating the informationfrom each of the detection systems to provide information concerning theDNA sequence.

The avidin sources 30S, 30A, 30G, 30C and 30T each contain avidinpuchased from known suppliers, with each avidin molecule in thepreferred embodiment combined with three fluorescein molecules. Theavidin sources are arranged to contact the DNA fragments and may be in asection of gel placed adjacent to the electrophoresis channel. In thiscase, this section of the gel should have a pH of approximately 8 toavoid movement of the three fluorescein-marked avidin byelectrophoresis. When the biotinylated DNA strands reach the section ofgel that has a pH of 8, they will pick up the fluoresceinated avidinwhich moves very slowly or is stationary in this section of the gel.

To prepare the second section of gel with fluoresceinated avidin, thefluoresceinated avidin may be electrophoresed from the exit end of thechannel inwardly. In this embodiment, it moves in this direction slowlybecause its pI is slightly higher than the pH of the second section ofgel. Alternatively, it may be mixed in gel.

Because the fluorescein-avidin-biotin-DNA complex molecule is acidic inthe pH 8 gel, it will continue to move out of this section of the gelwhere it is then passed to a sample volume within the detection systemby an eluant. The sequences of separation determined before theattachment of avidin are maintained and not substantially altered. Inthe alternative, the bands of DNA fragments may be electrophoresed intoa more mobile liquid containing fluorescein marked avidin forcombination with the avidin. The avidin binds selectively to the biotinattached to the ends of the DNA fragments and unreacted fluoresceinatedavidin is separated from the fluorescein-avidin-biotin-DNA complex bystandard techniques such as chromatography.

The detection systems each include an optical system for detecting thepresence or absence of bands and converting the detection of them toelectrical signals which are applied electrically to the correlationsystem 34 indicating the sequence of the fragments with respect to boththe standard fragments from the source of standard length DNA 18 and theA, G, C and T fragments from the containers 20A, 20G, 20C and 20Trespectively.

In FIG. 2, there is shown a simplified block diagram of anotherembodiment of DNA sequencing apparatus A10. This apparatus is similar tothe DNA sequencing apparatus 10 of FIG. 1 and the components areidentified in a similar manner with the reference numbers being prefixedby the letter A.

In this embodiment, instead of the containers for DNA and chemicaltreatment for A, G, C and T of the embodiment of DNA sequencing system10 shown at 20A, 20G, 20C and 20T in FIG. 1, the DNA sequencingapparatus A10 includes containers for treatment of the DNA in accordancewith the method of Sanger described by F. Sanger, S. Nicklen and A. R.Coulson, "DNA Sequencing with Chain-Terminating Inhibiters," Proceedingsof the National Academy of Science, USA, Vol. 74, No. 12, 5463-5467,1977, indicated in the embodiment A10 of FIG. 2 at A20A, A20G, A20C andA20T shown as a group generally at A12.

In this method, the strands are separated and used as templates tosynthesize DNA with synthesis terminating at given base types A, G, C ora T in a random manner so as to obtain a plurality of differentmolecular weight strands. The limited synthesis is obtained by usingnucleotides which will terminate synthesis and is performed in separatecontainers, one of which has the special A nucleotide, another thespecial G nucleotide, another the special C nucleotide and another thespecial T nucleotide. These special nucleotides may be dideoxynucleotides or marked nucleotides, both of which would terminatesynthesis. So, each of the four batches will be terminated at adifferent one of the types of bases A, G, C and T randomly.

In this embodiment, the fragments may be marked by biotin at one end inthe manner shown in FIG. 1. However, in the preferred embodiment of FIG.2, instead of labeling with biotin, the fragments are labeled by aninverted complementary repeat of DNA as shown at A11 before beingapplied to the channels indicated at A12 in FIG. 2. The design ofinverted complementary repeat takes advantage of the process ofdesigning small DNA fragments known as oligonucleotides. This process iswidely described in the literature as well as such patents asPhosphoramidite Components and Processes (U.S. Pat. No. 4,415,732), thedisclosure of which is incorporated herein.

After the electrophoresis, the inverted complementary repeat forms ahairpin from a palidrone of duplex DNA, which is then combined withethidium bromide and detected by the detection system using a wavelengthof light appropriate to the intercalated ethidium bromide rather thanwavelengths of light appropriate to the fluorescein marking. If one useshighly sensitive detection techniques, the inverted repeat would not beused and detection would be accomplished either by sensing ethidiumbromide that intercalated between portions of the unknown DNA thathappened to form duplex DNA, or by ethidium bromide that attached tosingle stranded DNA, or by the inherent fluorescence of the DNA itself.If one used radioactive markers, detection would be accomplished bysensing light given off by the combination of the radioactive marker andscintillation fluid.

In FIG. 3, there is shown a separating system which includes a slab ofgel 27 as known in the art with five sample dispensing tubes indicatedgenerally at 29A terminating in aligned slots 51 in the gel 27 on oneend, with such slots in contact with a negative potential buffer well 29having a negative electrode 47A, and five exit tubes at the other endlocated at 31A terminating in apertures in the gel 27, as well as apositive potential buffer well 31 having a positive electrode 53A. Thematerial to be electrophoresed is inserted into slots 51 through tubes29A and due to the field across the gel 27 moves from top to bottom inthe gel and into the appropriate corresponding exit tubes of the group31A. The gel slab 27 has glass plates 27A and 27B on either side toconfine the sample and gel. Buffer fluid from the buffer well 31 ispumped at right angles to the gel 27 from a source at 57 by pumpsconnected to tubes 31A to pull fluid therethrough. The buffer fluidpicks up any DNA that is electrophoresed into the exit holes 31A andmakes its way to sensing equipment to be described hereinafter or toprovide communication with other gel slabs for further electrophoresisof the DNA strands being electrophoresed from the slab 27.

In FIG. 4, there is shown another embodiment B26A of gel electrophoresishaving a negative-potential buffer for the A channel indicated generallyat B29A, a gel electrophoresis channel for A terminated DNA indicated atB27A and a positive potential buffer for the channel indicated at B31A.This embodiment is intended to provide a thin cylindrical gel for eachchannel so as to permit easier temperature control and thus alleviatechanges in migration rates by different temperatures such as may occuracross the slab 27 (FIG. 3). In addition, the field could be adjustedindependently for each channel to maintain proper synchronization.

For this purpose, the channel B27A includes a 0.5 to a 1 millimeterinner diameter pyrex column such as a chromatographic column indicatedat 33 with a gel inside of it. The gel is prepared by inserting it inthe column while unhardened and permitting it to harden. The column 33is of sufficient length to separate the DNA. Fluoresceinated avidin maybe initially electrophoresed upward from its exit end shown at 35 tomeet with DNA entering from its entrance end shown at 37 in theembodiment which uses biotin-avidin-fluorescein as a marker. The columnmay be temperature controlled by a conventional chromatographictemperature controlling apparatus 39 which is a glass casing about thecolumn which receives a liquid at one end such as at 41 and removes itat 43.

At one end of the channel B27A is the buffer B29A adapted to provide abuffer solution in a plexiglas surrounding cup shaped container 45,which buffer extends over the entering end 37 and contains within it anegative voltage electrode 47.

At the exit end, there is similarly mounted a buffer compartment 51containing buffer which is grounded by an electrode 53 and emerses inbuffer the exit end 35 of the gel column B27A. It may be made ofplexiglass and may be shaped with a reducing oriface ending in amicro-orifice 55 at its lower end to permit the flow of buffertherethrough containing DNA which emerged from exit end 35 for detectingwith a chopped light source as described above. To supply new buffer, abuffer reservoir 57 is connected through a pump 59 to the top of thebuffer 51, with the flow rates being designed to prevent a vortex nearthe exit end 35 and to permit a flow rate sufficient for optimumsignal-to-noise ratio of buffer. Another embodiment would transfer theflowing buffer-DNA solution to a flow-through cell for detection in aspectrofluorometer or specifically designed HPLC fluorescence detection.

In one embodiment, the buffer may include the ethidium bromide forexciting 300 or 390 nanometers and detecting at 590 nanometers.

In FIG. 5, there is shown still another embodiment C26A which may besubstituted for the column 33 and gel and includes capillary columns 61,63 and 65 as commonly used in capillary electrophoresis. These columnsmay be filled with buffer solution rather than a gel and be used forelectrophoresis. In such as case, several capillaries may be used as asubstitute for one column of the embodiment of FIG. 4. Thus, the sameband of A, G, C or T type bases might flow through several parallelbundles of capillaries, or they might flow through only one capillaryper type of base.

The separation path such as gel channels or capillary tube length shouldbe no longer than two meters for a range of lengths of DNA from 50 to10,000 or more bases. However, as the range of DNA lengths increase, thetime required increases. Also, the time required for each separation isin the range of from 1/2 second to 5 minutes for each added base oflength separation.

In FIG. 6 there is shown a block diagram of the detection system 32A.The detection systems 32S, 32G, 32C and 32T are substantially identicalto the detection system 32A and so only the system 32A will be describedin detail herein. The detection system 32A includes an electrophoresischannel 42, a sample volume 43, a light source 44 and an opticaldetection system 46.

In one embodiment avidin marked with fluorescein in thefluoresceimated-avidin source flows into the gel which receives A typeterminated strands from channel 26A on conduit 48 and is attached to thebiotinylated DNA fragments. Actually, the electrophoresis channel 42 maybe a gel section positioned at the end of the electrophoresis channel26A for continuous electrophoresing. After the fluoresceinated avidin isattached to the biotinylated DNA in the electrophoresis channel 42, thecomplex molecule is eluted out of the electrophoresis channel 42 intothe sample volume.

In another embodiment, the DNA may be marked by a palindrome describedabove and the detection system would then utilize a different wavelengthof light and would not require the fluorescenated-avidin source 40 butrather an ethidium bromide source 40A.

The sample volume 43 is irradiated by the light source 44. Light fromthe light source 44 is detected and converted to electrical signals bythe optical detection system 64 for application through the conductor50A to the correlation system 34 (FIG. 1). In one embodiment, thefluorescenated-avidin source 40 contains a fluorescent marker having aperiod of fluorescence sufficiently long compared to backgroundfluorescence in the DNA and associated materials to permit significantseparation of the signal from the fluorescence. In this embodiment, thedecay lifetime of the fluorescent marker should be at least ten timesthe duration of the background fluorescence which backgroundfluorescence is expressed in the form of noise in a detected signal.

Some known appropriate fluorescent markers are: (1) rare-earth organocomplexes, consisting of rare earth bound to organic compounds with theresulting complex having the desired properties, such as europium,benzoylacetonate and europium benzoyltrifluoroacetonate, as discussed byS. I. Weissman in the Journal of Chemical Physics, vol. 10, page214-217, 1942 incorporated by reference herein; (2) pyrenebuterate, asdiscussed by Knopp and Weber in the Journal of Biological Chemistry,vol. 242, p. 1353 (1963) and vol. 244, p. 3609 (1969) incorporated byreference herein; (3) fluorescein isothiocyanate (FITC); (4) rhodamineisothiocyanate (RITC); (5) tetramethylrhodamine isothiocyanate (TRITC);(6) phycoerythrin; and (7) their analogs and substituted derivatives.Such materials are available commercially such as from Research Organicsin Cleveland, Ohio. In the preferred embodiment, fluorescein avidin DSCis purchased from Vector Laboratories, Inc., 1429 Rollins Road,Burlingame, Calif. 94010.

The light source 44 includes a pulsed light source 52 and a modulator54. The pulsed light source 52 is selected to emit light within theabsorbance spectrum of the fluorescent marker. Since differentfluorescent markers may be used, this frequency differs from fluorescentmarker to fluorescent marker. Moreover, in one embodiment, the modulator54 controls the pulsed light source 52 to select intervals betweenpulses, the intervals being provided to permit the decay of fluorescentlight from the background fluorescent material, during which time thefluorescent light from the bound markers is measured.

These time periods between pulses are sufficiently long to encompass theentire delay period. This is done because the delay period of theattached fluorescent marker is relatively long compared to backgroundnoise fluorescence and so a period of time may pass before themeasurement is made by the optical detection system 46. Typically, thepulse of light has a duration of approximately three nanoseconds and thebackground fluorescence decay lasts for approximately ten nanosecondswhile the fluorescent marker attached to the avidin has a decay lifetimeof 100 nanoseconds.

Typically, the optical detection system 46 begins reading atapproximately 50 nanoseconds after the initiation of the excitationpulse from the laser and continues for approximately 150 nanosecondsuntil 200 nanoseconds after the initiation of the three nanosecondpulse. Although in the preferred embodiment, a pulsed laser light source52 is utilized, a broad band light source combined with filters or amonochrometer may be utilized to provide the narrow band in theabsorption spectrum of the marker.

Another embodiment uses an electro-optic modulator which modulates acontinuous light source at a frequency typically at 10 khz, withessentially 100% depth of modulation and 50% duty cycle. A pulsegenerator provides a signal both to the modulator via a driver and to alock-in amplifier as a reference signal. Another embodiment uses acontinuous light source with no modulations.

To detect the bands in the electrophoresis gel of the electrophoresischannel 42 indicating particular DNA fragments, the optical detectionsystem 46 includes certain viewing optics 60, a filter 62, and anoptical detection system 64. The filter 62 selects the frequency oflight transmitted through it by the viewing optics 60 which focuses thelight onto the optical detection system 64. The optical detection system64 is electrically connected to the modulator 54 to gate an electricalsignal to the conductor 50A indicating the presence or absence of a bandof DNA fragments in the electrophoresis channel 42.

The filter in the preferred embodiment includes an interference filterhaving a pass band corresponding to the high emission spectrum of thefluorescent marker. Such filters are known in the art and may bepurchased from commercial sources with bands to correspond to commonemission bands of fluorescent markers. In addition, there may belong-wavelength-passing interference filters and/or colored glassfilters. Another embodiment uses a monochrometer instead of a filter.

The viewing optics 60 consists of a lens system positioned injuxtaposition with filter 62 to focus light onto the optical detectionsystem 64. It may be any conventional optical system, and the opticaldetection system 64 should include a semiconductor detector or aphotomultiplier tube, such as the Model EMI 9798A made by EMI Gencon,Plainview, N.Y.

In the first embodiment, the output of the photomultiplier orsemiconductor is gated in response to the signals from the modulator 54to occur after a time delay after each pulse from the pulsed laser lightsource 52. For example, a time delay may be included before theelectrical signal is applied to an amplifier and thus provide anelectrical signal to the conductor 50A or to an amplifier, the output ofwhich is electrically connected to the conductor 50A. In the preferredembodiment, the time delay is 50 microseconds and the gate or amplifieris maintained open by a monostable multivibrator for approximately 150nanoseconds. In the second embodiment, the square wave output of amodulator is compared with the signal from the detector using a lock-inamplifier. In a third embodiment, no modulation is performed.

In FIG. 7 there is shown a block diagram of the correlation system 34having a standard channel input circuit 70S, a gating system 72, adecoder 74, a memory 76 and a read-out system 78. The standard channelinput circuit 70S is electrically connected to the OR gate 74S which iselectrically connected to the other channels A, G, C and T and to thegating system 72 which receives channel input signals from each of thechannels A, G, C, and T similar to that of channel 70S.

The gate 74S is electrically connected to the memory 76 which receivesignals from gate 74S indicating the presence of DNA fragments in theparticular one of the nucleic acid bases or in the standard channel. Thememory 76 is electrically connected to the read-out system 78 to printout the sequence.

The standard channel input circuit 70S includes a pulse shaper 82A, abinary counter 84S, a time delay 94S of the clock 80 and a latch 86 withthe input of the pulse shaper 82S being electrically connected to aconductor 50S and its output being connected to OR gate 74S through timedelay 94S and to the binary counter 84S. The output of the binarycounter 84S is connected to the latch 86 to provide a time incrementsignal to the latch 86, the output of which is applied to one of theinputs of memory 76 when triggered by a signal from OR gate 74S. Theconductor 50S corresponds to conductors 50A, 50G, 50C and 50T (FIG. 2)except that conductor 50S is the output for the standard clock channelrather than for adenine. guanine, cytosine or thymine.

The latch 86 and the decoder 74 are pulsed by a signal from the gate 74Sto write into the memory 76 for recording with a distinctive signalindicating a clock timing pulse which is later printed to indicate thetime that particular DNA segments have been received and detected in thedetection system 32A, 32G, 32C and 32T (FIG. 1). The binary counter 84Sreceives clock pulses from clock 80 to which it is connected and thuscontains a binary signal representing time for application to the latch86.

The switching circuit 72 includes a decoder 74 which is electricallyconnected to four inputs from channels 70A, 70G, 70C and 70Trespectively, for receiving signals indicating the presence of types A,G, C, and T, fragments as they appear on input conductors 50A, 50G, 50Cand 50T, (of FIG. 1 and FIG. 2). The signals on conductors 50A, 50G,50C, and 50T are each applied to respective ones of the pulse shapers82A, 82G, 82C, and 82T, the outputs of which are electrically connectedthrough corresponding ones of the conductors 92A, 92G, 92C, and 92T todifferent inputs of the decoder 74 and to inputs of the OR gate 74S, sothat the decoder 74 receives signals indicating the presence of a DNAfragment for application to the memory 76 upon receiving a signal onconductor 90S from the OR gate 74S. The OR gate 74S applies such asignal when receiving a calibration signal from the channel 70S or whenreceiving a signal from any one of the channels 70A, 70G, 70C, and 70T,so that the memory 96 receives calibration signals or signals indicatingDNA for reading out, after a delay within the memory 76, to the readoutsystem 78.

The OR gate 74S receives its calibration signal from channel 70S after adelay imparted in the delay line 74S, but does not have such acorresponding delay in channel 70A, 70G, 70C, and 70T. However, asimilar delay is within encoder 74 to be described hereinafter so thatthe appearance of DNA fragments will be sent to the memory 76 in a timeframe corresponding to that of the calibration signals from channel 70S.The output of the decoder 74 is electrically connected to the memory 76through a conductor 100.

In FIG. 8, there is shown a schematic circuit diagram of the decoder 74having a delay line 94, an OR gate 102 and a plurality of codingchannels 74A, 74G, 74C and 74T to respectively indicate fragmentsterminating with the bases, adenine, guanine, cytosine and thyminerespectively.

The channel 74A includes AND gate 106, having its inputs electricallyconnected to conductor 92A and 90A to receive on conductor 90A a clocksignal from the OR gate 74S (FIG. 7) and on its other input a signalindicating the presence of an adenine terminated fragment on conductor92A.

Channel 74G includes AND gate 108, AND gate 110 and delay line 112.Conductor 92G indicating a guanine terminated strand is electricallyconnected to the inputs of AND gate 108 and 110. The output of AND gate108 is connected to one of the inputs of OR gate 102 and the output ofAND gate 110 is electrically connected through delay line 112 to theinput of OR gate 102 to provide two pulses in succession to gate 102.Thus, channel 74A applies one out pulse from the output of AND gate 106to one of the inputs of OR gate 102, whereas channel 74G applies twopulses. In either case, the sequence of pulses indicates the presence ofa particular one of the types of DNA fragments A or G.

Similarly, the channel 74C includes AND gates 114, 116 and 118, eachhaving one of its two inputs electrically connected to conductor 92C and90C and the channel 74T includes AND gates 120, 122, 124 and 126, eachhaving one of its inputs electrically connected to conductor 92T and theother connected to 90T. The output from an AND gate 74C is electricallyconnected to an input of OR gate 102, the output of AND gate 116 iselectrically connected through a delay 128 to the input of OR gate 102,and the output of AND gate 118 is electrically connected through a delay130 longer than the delay 128 to an input of the OR gate 106. With thisarrangement, the presence of a DNA strand terminating with cytosineresults in three pulses to the OR gate 102.

The output of AND gate 120 is electrically connected to an input of theOR gate 102, the output of the AND gate 122 is electrically connectedthrough a delay 132 to an input of the OR gate 102, the output of ANDgate 124 is electrically connected through a delay 134 longer than thedelay 132 to an input of the OR gate 102 and the output of AND gate 126is electrically connected through a delay 136 longer than the delay 134to an input of the OR gate 102. In this manner, the presence of athymine-terminated fragment results in four signals in series to theinputs of OR gate 102. The out gate of OR gate 102 is applied through adelay 94 with a similar time delay as the delay 94S (FIG. 7) to theoutput conductor 100 so as to provide a coded signal indicating thepresence of a particular DNA group to the memory 76 (FIG. 7) coordinatedwith a time signal.

In the operation of sequencing DNA, DNA strands with bases above 100 innumber are first marked with biotin, separated in accordance to the sizeof the fragment and then marked with avidin marked with one or morefluorescent molecules. The bands are then detected by light with theread-out in the emission spectrum taking place a sufficient amount oftime after excitation in the emission spectrum to screen against noise.

To mark DNA fragments with biotin, cloned strands are prepared andcleaved into fragments after which they are first marked wtih biotin andthen divided into four aliquots. A standard source of DNA strands alsomarked with biotin and having different rates of migration underelectrophoresis forms additional calibration batches. The four batchesare each individually, randomly cleaved by a chemical treatment for adifferent one of adenine, guanine, cytosine and thymine bases. In thealternative, strands may be separated and used as templates forsynthesizing randomly to a selected base type. In either case, thestrands may be marked with biotin, marked with an inverted complementarysequence of DNA, marked with a radioactive marker or left unmarked.

To separate the fragments, the biotin marked fragments including anystandard ladder source are each individually electrophoresced throughgel in different channels or in different columns. The gel and the fieldmust be uniform, although reference channels reduce uniformityrequirements, and when a single slab is used to migrate severaldifferent samples, the channels must be kept separate and be centeredsufficiently around the field so that the potential for causing them tomigrate is uniform. Preferably, the pH of the gel for separation is7.5-8.

The DNA fragments separate in accordance with their length duringelectrophoresis. Thus, the fastest migrating fraction is the fragmentwhich is cut or synthesized to the first base closest to the marked endof the strand and, since the channels are separate, it is known whichbase A, G, C or T is the first one in the sequence from the channel.

The next band in time in the gel is the next cleavage point which is onebase longer than the first one since it encompasses both the first baseand the second one from the biotinylated end of the DNA strand.Similarly, the third fragment to form a band during electrophoresis willencompass the first three base units and so on.

Because a large number of bases are used, there is a larger number ofcleavage points and the density of fragments in each band is relativelylow. Thus, the gel and the field must be selected to provide a band ofsufficient width, high enough density and adequate separation fordetection. The gel slab is sufficienty long such that the first bands tobe moved completely through the gel are fully resolved while the lastbands are unresolved in a continuous process. More specifically, atleast 10 percent of the bands are resolved and electrophoresed throughthe gel while the least mobile bands are yet unresolved near theentrance end of the gel.

To provide light amplification for measurement of the low density bandsin one embodiment, the bands are electrophoresced into a region wherethey are mixed with avidin marked with one or more fluorescentmolecules. Because avidin is a large molecule and strongly attracted tothe biotin, the DNA fragments in each band, as they are moved into afluorescent-marked avidin region, are marked with avidin. After beingmarked with avidin such as indicated at 32S, 32A, 32G, 32C and 32T (FIG.1), they are each moved through a detection system such as the oneillustrated in FIG. 2.

Because the avidin molecules are large, a number of fluorescent markersare attached to the same avidin molecule thus providing adequatedetection. The fluorescent-marked fragments are then moved into thesample volume within the detection system where they maintain theirrelative order. The movement must be sufficiently rapid in the gel sothat minimum resolution is lost.

The bands are eluted into the sample volume where individual lightsources apply pulsed or chopped light within the optimum absorptionspectrum of the fluorescent marker or ethidium bromide marker in thesecond embodiment of approximately three nanoseconds duration. The lightis sensed by a detector approximately fifty nanoseconds after thebeginning of the three nanosecond pulse of light and the resultingelectrical signal is amplified. This light is detected and correlated toprovide the sequence of DNA in accordance with the channel as indicatedby a detector at the end of each of the detection systems.

As can be understood from the above description, the DNA sequencingsystem of this invention enables continuous sequencing and thus mayhandle in a continuous, automatic manner, a large number of bases. Thisis accomplished by the combination of continuous electrophorescing withthe amplification provided by the avidin attachment at the end of thefirst separation.

Although a preferred embodiment of the invention has been described withsome particularity, many modifications and variations are possible inthe preferred embodiment within the light of the above description.Accordingly, within the scope of the appended claims, the invention maybe practiced other than as specifically described.

What is claimed is:
 1. A method for sequencing DNA comprising the steps of:preparing a multiplicity of identical DNA strands; marking the DNA strands on one end with biotin; dividing the DNA strands into at least four batches; preparing at least one additional batch of DNA strands with known lengths to be electrophoresed as a time base; randomly cleaving at least some of the DNA strands in a first batch at adenine bases by a chemical treatment to form an adenine-strand batch; randomly cleaving at least some of the DNA strands in a second batch at guanine bases by a chemical treatment to form a guanine-strand batch; randomly cleaving at least some of the DNA strands in a third batch at cytosine bases by a chemical treatment to form a cytosine-strand batch; randomly cleaving at least some of the DNA strands in a fourth batch at thymine bases by a chemical treatment to form a thymine-strand batch; applying samples from each of the four batches after cleaving at their respective bases to four identical channels of gel electrophoresis apparatus; applying at least one time base source of DNA of known length strands to at least one additional channel positioned among the aforementioned four channels whereby a reference time of electrophoresing may be obtained from said channel; electrophoresing the DNA strands through a gel slab so that bands of more mobile strands, each of which has substantially uniform length strands, are fully resolved while the less mobile strands to be later formed into bands are unresolved in a continuous process such that at least ten percent of the bands are fully resolved and electrophoresed through the gel while the less mobile strands are yet unresolved into bands in the gel; whereby a plurality of adenine-strand bands is formed in a first channel in an order corresponding to the length of the strands; a plurality of guanine-strand bands is formed in a second channel in an order corresponding to the length of the strands; a plurality of cytosine-strand bands is formed in a third channel in an order corresponding to the length of the strands; and a plurality of thymine-strand bands is formed in a fourth channel in an order corresponding to the length of the strands; attaching fluorescent markers to avidin whereby the fluorescently labeled avidin is combined with the biotin markers at the end of the DNA strands; moving the fluorescently-marked bands in sequence through a medium; scanning said bands with laser having a narrow bandwidth substantially comforming to the optimum absorption spectrum of the fluorescent markers; pulsing the laser light with pulses of shorter duration than three nanoseconds during a first period of time; detecting the fluorescent emission from the markers across a narrow selective band of wavelengths conforming substantially to the optimum emission spectrum of the markers during a second period of time; said second period of time beginning at least fifty nanoseconds from the start of its corresponding pulse of laser light and terminating at a time no greater than one hundred fifty nanoseconds from the start of the pulse of the laser light; and identifying and recording the sequence of the bands in each of the channels so as to indicate the DNA sequence.
 2. A method for sequencing DNA comprising the steps of:preparing a multiplicity of identical DNA strands; creating DNA strands with random terminations at adenine bases and with random lengths to form an adenine-strand batch; creating DNA strands with random terminations at guanine bases and with random lengths to form a guanine-strand batch; creating DNA strands with random terminations at cytosine bases and with random lengths to form a cytosine-strand batch; creating DNA strands with random terminations at thymine bases and with random lengths to form a thymine-strand batch; applying a sample from a different one of each of the four batches to a corresponding one of four identical channels of separating apparatus; separating the DNA strands into bands within the channels so that the bands of more mobile strands in the channels are fully resolved while some of the DNA strands to be later formed into bands are unresolved in a continuous process such that at least ten percent of the bands are resolved while the less mobile DNA strands are yet unresolved into bands in the channel; fluorescently marking the DNA strands; and applying light to the resolved bands while the bands are still moving with respect to each other in the continuous process and identifying and recording the sequence of the bands in each of the channels from a response of the fluorescently marked DNA strands to the applied light, so as to indicate the DNA sequence.
 3. A method according to claim 2 in which the step of separating the DNA strands into bands includes the substeps of:forming a plurality of adenine-strand bands in a first channel in an order corresponding to the length of the strands; forming a plurality of guanine-strand bands in a second channel in an order corresponding to the length of the strands; forming a plurality of cytosine-strand bands in a third channel in an order corresponding to the length of the strands; and forming a plurality of thymine-strand bands in a fourth channel in an order corresponding to the length of the strands.
 4. A method according to claim 2 in which the step of separating includes the step of separating the DNA strands into bands by gel electrophoresis.
 5. A method according to claim 2 in which the step of separating includes the step of separating the DNA strands into bands by HPLC.
 6. A method according to claim 2 in which the step of creating an adenine-strand batch includes the step of cleaving the strands at adenine bases whereby DNA strands having random lengths are created.
 7. A method according to claim 3 in which the step of creating an adenine-strand batch includes the substep of using some of the multiplicity of identical DNA strands as templates for synthesizing DNA strands of random lengths whereby randomly terminated adenine bases are created.
 8. A method according to claim 2 including the step of applying at least one time base source of DNA to at least one channel positioned among the aforementioned four channels whereby a reference time of electrophoresing may be obtained.
 9. A method according to claim 2 in which the step of applying light to the resolved bands and recording the sequence of bands includes the substep of fluorescently marking the DNA strands on one end with fluorescently labeled biotin before separating.
 10. A method according to claim 9 in which the step of identifying further includes the steps of:moving the fluorescently-marked bands in sequence through a medium; scanning said bands with laser light having a narrow bandwidth substantially conforming to the optimum absorption spectrum of the fluorescent markers; detecting the fluorescent emission from the marker across a narrow selective bandwidth conforming substantially to the optimum emission spectrum of the markers; and identifying and recording the sequence of the bands in each of the channels so as to indicate the sequence of DNA fragments.
 11. A method according to claim 10 in which the step of scanning said bands with laser light includes the step of pulsing the laser light with pulses of shorter duration than three nanoseconds; and the step of detecting the fluorescent emission from the marker includes the step of detecting the fluorescent emission through a second period of time beginning at least fifty nanoseconds from the start of its corresponding pulse of laser light and terminating at a time no greater than one hundred fifty nanoseconds from the start of the pulse of the laser light.
 12. A method for sequencing DNA comprising the steps of:preparing a multiplicity of identical DNA strands; preparing, from the multiplicity of identical DNA strands, fluorescently marked DNA strands with random lengths terminated at least at one of different ones of the adenine base, guanine base, cytosine base and thymine base, wherein at least one batch of DNA strands is formed, which batch is terminated at one of the adenine base, guanine base, cytosine base and thymine base; applying samples of the fluorescently marked DNA strands to at least one channel of a separating apparatus; separating the strands within at least one channel so that the bands of more mobile strands in the channels are fully resolved while some of the less mobile strands to be later formed into bands are unresolved in a continuous process such that at least ten percent of the bands are fully resolved while the less mobile strands are yet unresolved into bands in the channel; applying light from a source of light to the resolved bands; and identifying and recording the sequence of the bands in the channel from the reaction of the fluorescently marked strands with the light so as to indicate the DNA sequence.
 13. A method according to claim 12 in which the step of preparing marked DNA strands from the multiplicity of identical DNA strands includes the step of terminating the strands during synthesis at different ones of the adenine base, guanine base, cytosine base and thymine base.
 14. A method according to claim 12 in which the step of separating includes the step of separating the strands by HPLC.
 15. A method according to claim 12 in which the step of preparing marked DNA strands from the multiplicity of identical DNA strands includes the step of randomly cleaving the DNA strands at least at one of the adenine base, guanine base, cytosine base and thymine base whereby DNA strands are created with random lengths.
 16. A method according to claim 12 in which the step of preparing includes the steps of:fluorescently marking the DNA strands on one end; dividing the DNA strands into at least four batches; and cleaving the DNA strands in each of the four batches at different one of adenine, guanine, cytosine and thymine.
 17. A method according to claim 12 in which the step of identifying includes the steps of:moving the bands in sequence through a medium; scanning said bands with laser light having a narrow bandwidth substantially conforming to the optimum absorption spectrum of the fluorescent markers; detecting the fluorescent emission from the marker across a narrow selective bandwidth conforming substantially to the optimum emission spectrum of the markers; and identifying and recording sequence of the bands in each of the channels so as to indicate the sequence of DNA fragments.
 18. A method according to claim 17 in which the step of scanning said bands with laser light includes the step of pulsing the laser light with pulses of shorter duration than three nanoseconds; and the step of detecting the fluorescent emission from the marker includes the step of detecting the fluorescent emission through a second period of time beginning at least fifty nanoseconds from the start of its corresponding pulse of laser light and terminating at a time no greater than one hundred fifty nanoseconds from the start of the pulse of the laser light. 